There has been known a technique in which an NOx storage reduction catalyst (hereinafter referred to as an NSR catalyst) is disposed in an exhaust passage of an internal combustion engine. This NSR catalyst serves to occlude or store NOx contained in an incoming exhaust gas when the oxygen concentration of the exhaust gas is high, and to reduce the occluded or stored NOx when the oxygen concentration of the incoming exhaust gas becomes low and when a reducing agent exists.
The sulfur oxides (SOx) generated by the combustion of sulfur components included in fuel are occluded or stored in this NSR catalyst, similar to NOx. The SOx stored in this manner is more difficult to be released than NOx, and is accumulated into the NSR catalyst. This is referred to as sulfur poisoning. An NOx purification (i.e., removal and/or reduction) rate in the NSR catalyst is decreased due to this sulfur poisoning, and hence, it is necessary to carry out sulfur poisoning recovery treatment at appropriate times. This sulfur poisoning recovery treatment is carried out by circulating the exhaust gas, of which the oxygen concentration has been made low, through the NSR catalyst, with the temperature of the NSR catalyst being made high.
Thus, when the oxygen concentration is reduced to become a rich air fuel ratio at the time of recovery of sulfur poisoning, H2S may flow downstream of the NSR catalyst, so that a nasty smell may occur.
Here, there has been known a technique which suppresses the discharge of H2S by carrying out a lean spike in which an air fuel ratio is adjusted to a lean air fuel ratio only in a short period of time at a predetermined point in time, while making a target air fuel ratio to be a rich air fuel ratio at the time of sulfur poisoning recovery treatment (for example, refer to a first patent document). In this technique, the interval or duration in which the lean spike is carried out is made longer in accordance with the decreasing amount of deposit or storage of sulfur components.
In addition, there has also been known another technique of oxidizing H2S by supplying secondary air at the time of sulfur poisoning recovery treatment (for example, refer to a second patent document).
Moreover, there has also been known a further technique in which air is supplied to a catalyst by opening a throttle valve during a deceleration fuel cut-off operation, so that the catalyst is put into an oxidized state, thereby suppressing the generation of H2S (for example, refer to a third patent document).
Further, there has also been known a technique of suppressing the generation of H2S by making longer the period of time of a fuel cut-off operation (for example, refer to a fourth patent document).
Furthermore, there has also been known a technique in which in cases where an exhaust gas purification catalyst is in a state to generate a nasty smell at the time of carrying out fuel cut-off control, the flow rate of air flowing into the exhaust gas purification catalyst is controlled to become larger than that at the time of idling (for example, refer to a fifth patent document).
However, an NOx selective reduction catalyst (hereinafter also referred to as an SCR catalyst) can be provided at the downstream side of an NSR catalyst or a three-way catalyst in which sulfur poisoning occurs. This SCR catalyst is a catalyst which serves to carry out selective reduction of NOx by means of a reducing agent. Then, sulfur poisoning may be caused in the SCR catalyst by the H2S which flows out of the NSR catalyst. Here, in this conventional technique, no mention is made to sulfur poisoning recovery treatment of both the catalysts in the case of the SCR catalyst being provided at the downstream side of the NSR catalyst. For this reason, there is a fear that sulfur poisoning recovery may not be carried out in an appropriate manner. For this reason, there is a fear that H2S may be released into the atmospheric air, thereby giving off a nasty smell. In addition, the removal or reduction rate of NOx may also be decreased.